1. Field of the Invention
This invention relates generally to a high ratio multilaced raster scan system for a CRT and, more particularly, to a system for providing a high ratio multilaced raster scan on a CRT in order to obscure the spaces between the raster lines when making a hard copy from the screen of the CRT.
2. Description of the Prior Art
As is well known, photographs or hard copies are often made from electronic images displayed on the screen of a CRT. As is also well known, CRT's operate to convert video signals to electronic images, and the video signals may originate from any variety of sources such as a conventional television receiver, a video tape, or a computer from which the image was generated in a well-known manner. Since electronic images displayed on the screen of the CRT are transitory in that they exist only so long as the CRT is operative, it is often desirable to record such electronic images on a hard copy such as a photograph. Each electronic image displayed on the screen of the CRT, however, is composed from a plurality of horizontally spaced apart raster lines presented at a rate of 60 fields to 30 frames per second. In the United States, the raster lines of two fields forming each frame are spatially interlaced to produce a composite frame of 525 horizontal lines. In making a photograph or hard copy from the screen of such a CRT, however, the blank spaces between the horizontal raster lines are clearly visible in the photograph, and thus it has been suggested that the raster lines be further interlaced to obscure the visibility of the blank spaces between the horizontal raster lines in any hard copy made from the screen of the CRT. Toward this end, systems have been developed in which the succeeding vertical sync pulses of the vertical sync pulse train signal are phase shifted with respect to each other in order to shift the whole raster downwardly in repeatable numbers of increments. Such systems, however, generally involve delay lines and other complex and costly circuitry.
In U.S. Pat. No. 4,251,835, entitled "Image-Forming System for Obscuring Poster Lines", by Joern B. Eriksen, issued Feb. 17, 1981, there is disclosed a system for doubling the interlace in a CRT raster in order to obscure the blank spaces between the horizontal raster lines in a hard copy made from the screen of the CRT by impressing a DC bias voltage to the vertical ramp pulse train signal so as to vertically shift the horizontal raster lines without introducing a phase shift to succeeding pulses in either the vertical sync pulse train or the vertical ramp pulse train signals. This system appears to be simpler and less costly than the aforementioned systems which require circuits for injecting a phase shift to succeeding vertical sync pulses. However, it also has only a limited capability in that it can only double the interlace and cannot provide higher multiples of interlace raster scans. In addition, although the offsetting DC bias voltage which is combined with the vertical ramp pulse train signal might be calibrated by simply viewing the screen of the CRT since there is only provided a doubling of the interlace raster scan, such calibration would become virtually impossible in a higher ratio multiple interlace system.
Therefore, it is a primary object of this invention to provide a high ratio multiple interlace raster scan system for a CRT in order to obscure the visibility of the blank spaces between the horizontal raster lines of the CRT by combining a DC biasing or offsetting voltage with the vertical ramp pulse train signal without offsetting the phase relationship between succeeding vertical ramp pulses.
It is a further object of this invention to provide a high ratio multiple interlace raster scan system for a CRT wherein the visibility of the blank spaces between the horizontal raster lines are minimized in any hard copy made from the screen of the CRT and there are also included simple means for calibrating the system so that the high multiple interlace raster lines are uniformly spaced apart across the screen of the CRT.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter. The invention accordingly comprises a mechanism and system possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure.